.TH std::result_of,std::invoke_result 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::result_of,std::invoke_result \- std::result_of,std::invoke_result

.SH Synopsis
   Defined in header <type_traits>
   template< class >

   class result_of; // not defined            \fI(since C++11)\fP
                                          \fB(1)\fP (deprecated in C++17)
   template< class F, class... ArgTypes >     (removed in C++20)

   class result_of<F(ArgTypes...)>;
   template< class F, class... ArgTypes > \fB(2)\fP \fI(since C++17)\fP
   class invoke_result;

   Deduces the return type of an INVOKE expression at compile time.

   F must be a callable type, reference to function, or reference to      \fI(since C++11)\fP
   callable type. Invoking F with ArgTypes... must be a well-formed       \fI(until C++14)\fP
   expression.
   F and all types in ArgTypes can be any complete type, array of unknown \fI(since C++14)\fP
   bound, or (possibly cv-qualified) void.

   If the program adds specializations for any of the templates described on this page,
   the behavior is undefined.

.SH Member types

   Member type Definition
               the return type of the Callable type F if invoked with the arguments
               ArgTypes....
   type        Only defined if F can be called with the arguments ArgTypes... in
               unevaluated context.
               \fI(since C++14)\fP

.SH Helper types

   template< class T >                                            \fI(since C++14)\fP
   using result_of_t = typename result_of<T>::type;           \fB(1)\fP (deprecated in C++17)
                                                                  (removed in C++20)
   template< class F, class... ArgTypes >
   using invoke_result_t = typename invoke_result<F,          \fB(2)\fP \fI(since C++17)\fP
   ArgTypes...>::type;

.SH Possible implementation

 namespace detail
 {
     template<class T>
     struct is_reference_wrapper : std::false_type {};
     template<class U>
     struct is_reference_wrapper<std::reference_wrapper<U>> : std::true_type {};

     template<class T>
     struct invoke_impl
     {
         template<class F, class... Args>
         static auto call(F&& f, Args&&... args)
             -> decltype(std::forward<F>(f)(std::forward<Args>(args)...));
     };

     template<class B, class MT>
     struct invoke_impl<MT B::*>
     {
         template<class T, class Td = typename std::decay<T>::type,
             class = typename std::enable_if<std::is_base_of<B, Td>::value>::type>
         static auto get(T&& t) -> T&&;

         template<class T, class Td = typename std::decay<T>::type,
             class = typename std::enable_if<is_reference_wrapper<Td>::value>::type>
         static auto get(T&& t) -> decltype(t.get());

         template<class T, class Td = typename std::decay<T>::type,
             class = typename std::enable_if<!std::is_base_of<B, Td>::value>::type,
             class = typename std::enable_if<!is_reference_wrapper<Td>::value>::type>
         static auto get(T&& t) -> decltype(*std::forward<T>(t));

         template<class T, class... Args, class MT1,
             class = typename std::enable_if<std::is_function<MT1>::value>::type>
         static auto call(MT1 B::*pmf, T&& t, Args&&... args)
             -> decltype((invoke_impl::get(
                 std::forward<T>(t)).*pmf)(std::forward<Args>(args)...));

         template<class T>
         static auto call(MT B::*pmd, T&& t)
             -> decltype(invoke_impl::get(std::forward<T>(t)).*pmd);
     };

     template<class F, class... Args, class Fd = typename std::decay<F>::type>
     auto INVOKE(F&& f, Args&&... args)
         -> decltype(invoke_impl<Fd>::call(std::forward<F>(f),
             std::forward<Args>(args)...));
 } // namespace detail

 // Minimal C++11 implementation:
 template<class> struct result_of;
 template<class F, class... ArgTypes>
 struct result_of<F(ArgTypes...)>
 {
     using type = decltype(detail::INVOKE(std::declval<F>(), std::declval<ArgTypes>()...));
 };

 // Conforming C++14 implementation (is also a valid C++11 implementation):
 namespace detail
 {
     template<typename AlwaysVoid, typename, typename...>
     struct invoke_result {};
     template<typename F, typename...Args>
     struct invoke_result<
         decltype(void(detail::INVOKE(std::declval<F>(), std::declval<Args>()...))),
             F, Args...>
     {
         using type = decltype(detail::INVOKE(std::declval<F>(), std::declval<Args>()...));
     };
 } // namespace detail

 template<class> struct result_of;
 template<class F, class... ArgTypes>
 struct result_of<F(ArgTypes...)> : detail::invoke_result<void, F, ArgTypes...> {};

 template<class F, class... ArgTypes>
 struct invoke_result : detail::invoke_result<void, F, ArgTypes...> {};

.SH Notes

   As formulated in C++11, the behavior of std::result_of is undefined when
   INVOKE(std::declval<F>(), std::declval<ArgTypes>()...) is ill-formed (e.g. when F is
   not a callable type at all). C++14 changes that to a SFINAE (when F is not callable,
   std::result_of<F(ArgTypes...)> simply doesn't have the type member).

   The motivation behind std::result_of is to determine the result of invoking a
   Callable, in particular if that result type is different for different sets of
   arguments.

   F(Args...) is a function type with Args... being the argument types and F being the
   return type. As such, std::result_of suffers from several quirks that led to its
   deprecation in favor of std::invoke_result in C++17:

     * F cannot be a function type or an array type (but can be a reference to them);
     * if any of the Args has type "array of T" or a function type T, it is
       automatically adjusted to T*;
     * neither F nor any of Args... can be an abstract class type;
     * if any of Args... has a top-level cv-qualifier, it is discarded;
     * none of Args... may be of type void.

   To avoid these quirks, result_of is often used with reference types as F and
   Args.... For example:

 template<class F, class... Args>
 std::result_of_t<F&&(Args&&...)> // instead of std::result_of_t<F(Args...)>, which is wrong
     my_invoke(F&& f, Args&&... args)
     {
         /* implementation */
     }

.SH Notes

       Feature-test macro      Value    Std                  Feature
   __cpp_lib_result_of_sfinae 201210L \fI(C++14)\fP std::result_of and SFINAE
   __cpp_lib_is_invocable     201703L \fI(C++17)\fP std::is_invocable, std::invoke_result

.SH Examples


// Run this code

 #include <iostream>
 #include <type_traits>

 struct S
 {
     double operator()(char, int&);
     float operator()(int) { return 1.0; }
 };

 template<class T>
 typename std::result_of<T(int)>::type f(T& t)
 {
     std::cout << "overload of f for callable T\\n";
     return t(0);
 }

 template<class T, class U>
 int f(U u)
 {
     std::cout << "overload of f for non-callable T\\n";
     return u;
 }

 int main()
 {
     // the result of invoking S with char and int& arguments is double
     std::result_of<S(char, int&)>::type d = 3.14; // d has type double
     static_assert(std::is_same<decltype(d), double>::value, "");

     // std::invoke_result uses different syntax (no parentheses)
     std::invoke_result<S,char,int&>::type b = 3.14;
     static_assert(std::is_same<decltype(b), double>::value, "");

     // the result of invoking S with int argument is float
     std::result_of<S(int)>::type x = 3.14; // x has type float
     static_assert(std::is_same<decltype(x), float>::value, "");

     // result_of can be used with a pointer to member function as follows
     struct C { double Func(char, int&); };
     std::result_of<decltype(&C::Func)(C, char, int&)>::type g = 3.14;
     static_assert(std::is_same<decltype(g), double>::value, "");

     f<C>(1); // may fail to compile in C++11; calls the non-callable overload in C++14
 }

.SH Output:

 overload of f for non-callable T

.SH See also

   invoke                 invokes any Callable object with given arguments
   invoke_r               and possibility to specify return type
   \fI(C++17)\fP                (since C++23)
   (C++23)                \fI(function template)\fP
   is_invocable
   is_invocable_r         checks if a type can be invoked (as if by std::invoke) with
   is_nothrow_invocable   the given argument types
   is_nothrow_invocable_r \fI(class template)\fP
   \fI(C++17)\fP
   declval                obtains a reference to its argument for use in unevaluated
   \fI(C++11)\fP                context
                          \fI(function template)\fP
